It will take more work to figure out if hydroxychloroquine -- which is sold generically and by the brand names Plaquenil and Quineprox -- truly guards against diabetes, write the researchers.

They included Mary Chester Wasko, MD, MSc, of the University of Pittsburgh's division of rheumatology and clinical immunology.

Rheumatoid Arthritis Study

Wasko and colleagues studied some 4,900 rheumatoid arthritis adult patients. None of the patients had been diagnosed with diabetes when the study started.

The study lasted 21 years. During that time, the patients noted their rheumatoid arthritismedications, including hydroxychloroquine.

Why would rheumatoid arthritis patients take a malaria drug? Hydroxychloroquine is taken to ease RA-related pain and inflammation. Other, newer RA drugs are also available.

In Wasko's study, about 1,800 patients had ever taken hydroxychloroquine.

Over the years, 54 patients who had ever taken hydroxychloroquine reported being newly diagnosed with diabetes, compared with 171 who had never taken hydroxychloroquine.

Less Risk of Rheumatoid Arthritis?

After considering various diabetes risk factors, the researchers found that patients who ever took hydroxychloroquine were 38% less likely to report a new diabetes diagnosis than those who never took hydroxychloroquine.

Patients who took hydroxychloroquine for more than four years were 77% less likely to report a new diabetes diagnosis than those who never took the antimalarial drug.

The study was purely observational. That is, the researchers didn't ask any of the patients to take hydroxychloroquine. So the results don't prove that the antimalarial drug warded off diabetes.

However, Wasko's team notes that previous studies suggest that hydroxychloroquine may help the body make and respond properly to insulin, the hormone that controls blood sugar.

The study doesn't show exactly what type of diabetes the patients developed. But most cases were probably type 2 diabetes, since that's the most common type of diabetes in adults, note the researchers.

Hydroxychloroquine is inexpensive and safe, so further studies should be done to see if the drug will lower diabetes risk in RA patients and the general population, note Wasko and colleagues.